Re: [Bloat] Random idea in reaction to all the discussion of TCP flavours - timestamps?

[Jonathan Morton <chromatix99@gmail.com>]

On 16 Mar, 2011, at 7:41 am, Fred Baker wrote:

> The question isn't "what is the magic mean queue depth for min-threshold to be set to"; it's "what mark/drop rate is sufficient to keep the queue somewhat shallow most of the time".

And that's what BLUE (and SFB) tries to do.  If the queue grows beyond some limit, it increments the marking rate.  If it becomes empty, it decrements it.  The result is an I-type control loop which has a reasonable chance of finding a steady state, if there is one.  SFB does it on a per-flow basis.

I also thought of an analogy just now, as I was playing with my train simulator - many people like car analogies, I prefer railway ones.  A router is like a bunch of railways meeting at a grand-union flying junction (typically implemented as a cloverleaf in the real world).  The more expensive kinds are built with wider curves that let trains go fast even in the junction.

You can have lightweight, fast passenger trains, running loaded in both directions, and these are your VoIP traffic.  Among them you might have heavy, slow freight trains, which just happen to weigh about 1500 tons each, but which run empty from the power station back to the mines.  You don't want to be on a passenger train stuck behind a freight, so railways build extra tracks, either at intervals or continuously, to keep freight trains in and allow passenger trains past.

But a railway can only carry one train on each track at a time, and tracks are expensive.  So sometimes a train still has to wait for another one.  They can simply wait one behind the other at signals, or the railways might decide to put a marshalling yard in at the junction, so that many trains can be stored and rearranged for efficient prioritisation.

Meanwhile, a wireless network is more like a bunch of railways which meet at the cheapest, skinniest single-track junction the builders could devise - only one train at a time can use it, and sometimes they even fall off the rails and have to be crowbarred back on.  It's a bit of a mess, but the junction is up in the mountains so it's very difficult to improve it.

The problem is that the railway company doesn't like to admit that the trains are slow and unreliable at this junction, so it employs lots of men with crowbars, and tries to avoid the subject when trains arrive several hours late.  Yet people *do* notice, especially during the holiday season when *everyone* is travelling and the freight trains are chock-full of parcels - and the snow is just starting to fall in earnest, which freezes the points.

 - Jonathan